Patent Application
20120000316
The invention relates to a method for producing pressed articles containing coal particles, to the pressed articles obtained in this way and to use of the pressed articles in methods for producing pig iron in a fixed bed or in methods for producing carbon carriers for methods for producing pig iron in a fixed bed.
Pressed articles containing coal particles, for example briquettes, that are used in methods for producing pig iron in a fixed bed, for example in melter gasifiers, or in methods for producing carbon carriers for methods for producing pig iron in a fixed bed, for example producing coke for blast furnaces, must have a certain shatter strength and compressive strength after discharge from the press. The shatter strength is required in order that the original size of the pressed articles is retained as far as possible in the course of charging into a process, irrespective of unavoidable falls, for example during the transfer from one conveyor belt to another, or during charging into a material bunker. The compressive strength is required in order that the original size of the pressed articles is retained after charging into a material bunker or a fixed-bed reactor, in spite of a pressure exerted by superposed layers of material. These strength requirements are also subsumed under the term green strength. Apart from the green strength, the hot strength of pressed articles is a criterion for their suitability for use—particularly when they are used in thermal processes. In the special case of using pressed articles containing fine-grained coal particles in methods for producing pig iron, such as for example in a melter gasifier or blast furnace, the term hot strength concerns a) a strength of the semicoke or coke particles remaining after pyrolysis of the pressed articles in a high temperature zone, and b) a strength of these semicoke or coke particles following chemical attack of a hot, CO2-containing gas. A minimum amount of hot strength makes it possible for the size of these particles that they have after the conversion of the pressed articles by pyrolysis into semicoke or coke particles to be largely retained. In the case of methods for producing pig iron in a fixed bed, the development of undersized material from pressed articles or coke particles before charging into a fixed bed or within a fixed bed is undesired because it worsens the permeability of the fixed bed. In the special case of a method for producing pig iron, this concerns both the gas permeability and the drainage behavior of the fixed bed with respect to the liquid pig iron and the slag. If the permeability of the fixed bed worsens, adverse effects are likely on its productivity, its specific energy requirement and its product quality.
It is known from WO 02/50219A1 to produce pressed articles with sufficient green strength from fine-grained coal particles by means of a binder system of quicklime and molasses. This involves mixing fine-grained coal particles of fine coal and quicklime, leaving the mixture to rest for the purpose of progressing the slaking reaction with moisture from the coal particles, then mixing in molasses, kneading the mixture thereby obtained and finally pressing pressed articles from it.
There are coals that exhibit extremely high absorptive capacity for water, particularly characterized by a high inherent moisture content. For use in the production of pig iron, however, the moisture content of the pressed articles should not be too high, that is to say at most 7% by weight. This is because, when using the pressed articles to produce pig iron or to produce carbon carriers for methods for producing pig iron, this moisture is a drain on energy, since the specific consumption of carbon carriers increases significantly with the moisture content of the pressed articles. Therefore, coals of a higher moisture content must be dried before being processed into pressed articles. In addition to the unwetted pore volume that already exists in the undried coal, additional pore volume is produced by driving out water from cavities during drying. The unwetted pore volume can absorb a corresponding amount of water or aqueous media. The additional pore volume can of course also once again absorb water or aqueous medium. Moreover, certain coals also have a tendency—particularly during intensive drying—to generate additional pore volume as a result of grain damage. When drying a coal with a high absorptive capacity for water to an acceptable moisture content before application of the method described in WO 02/50219A1 for producing pressed articles, a large additional pore volume is generated. Therefore, a dried coal particle sucks into its pores a considerable part of the molasses that are required for producing a bond on the particle surface and can be regarded as an aqueous solution. Therefore, for such coals with conventionally used additions of molasses of ≦10% by weight, with respect to the weight of the coal to be processed, sufficient strength of the pressed articles cannot be achieved. In order to allow pressed articles with sufficient strength nevertheless to be produced on the basis of a molasses binder, it is necessary
These measures are undesired, however, for reasons of process economy.
Also in the case of coals that are naturally less moist, which do not have to be dried to achieve a moisture content of the pressed articles of at most 7% by weight, part of the molasses is sucked into pores of the coal particles. However, molasses contain components which act catalytically with regard to a reaction of carbon with hot, CO2-containing gases, whereby the extent of a reaction of solid carbon with CO2 according to the Boudouard reaction increases, particularly in the hot zones of a fixed bed serving for producing pig iron at temperatures >800-1000° C., dependent on the pressure. As a result of this, the hot strength of semicoke or coke particles obtained by pyrolysis from pressed articles treated with molasses decreases.
The use of bitumen as a binder, proposed in WO9901583A1, does not give rise to such problems associated with molasses. However, production of pressed articles with bitumen entails very high binder costs.
The use of an aqueous bitumen emulsion as a binder system, proposed in AT005765U1, lowers the bitumen consumption by up to more than 50%. In practice, however, it has been found that the charging coals must have moisture contents of much more than 5% by weight in order for stable pressed articles to be obtained when using such bitumen emulsions. Moreover, there is the problem that pores present in the coal particles can absorb aqueous bitumen emulsion, or extract water from the emulsion and destabilize the latter as a result of droplet coalescence, before a largely uniform distribution of the emulsion within the material to be processed into pressed articles, and correspondingly uniform wetting of the particle surface by the emulsion, can take place. As a result, the effectiveness of the emulsion as a binder is reduced.
According to various embodiments, a method for producing pressed articles can be provided in which these disadvantages of the prior art are overcome, and pressed articles with sufficient green strength and hot strength can be produced even when using coal particles that have to be pre-dried, using an amount of a water-containing binder system that is less in comparison with known methods.
According to an embodiment, in a method for producing a pressed article containing coal particles in which the coal particles are mixed with a water-containing binder system and the mixture thereby obtained is further processed into pressed articles by pressing, before the mixing with the water-containing binder system, the coal particles are subjected to an impregnating step in which they are impregnated with a substance.
According to a further embodiment, the impregnating step may comprise damping the coal particles with the substance, spraying the coal particles with the substance, mixing the substance into a moving packed bed of the coal particles, or mixing the substance into a fluidized bed of the coal particles. According to a further embodiment, the substance with which the coal particles are impregnated in the impregnating step can be water. According to a further embodiment, the substance with which the coal particles are impregnated in the impregnating step can be a water-insoluble and/or water-repellent substance. According to a further embodiment, the substance with which the coal particles are impregnated in the impregnating step can be an aqueous solution of a substance or a substance mixture. According to a further embodiment, the substance with which the coal particles are impregnated in the impregnating step can be an aqueous suspension of solid colloids, the solid substance having water-repelling properties. According to a further embodiment, the substance with which the coal particles are impregnated in the impregnating step can be an emulsion containing water on the one hand and carbon-containing substances on the other hand. According to a further embodiment, the lower limit of the amount of substance added in the impregnating step can be 0.5% by weight, preferably 1% by weight, and the upper limit can be 5% by weight, preferably 3% by weight, particularly preferably 2% by weight, with respect to the weight of the material to be processed into pressed articles, the coal particles. According to a further embodiment, the binder system may contain molasses and quicklime or hydrated lime. According to a further embodiment, the binder system may contain an emulsion of bitumen in water. According to a further embodiment, iron- or iron-oxide-containing particles can also be processed in a mixture with the coal particles. According to a further embodiment, the pressed article can be subjected to a heat treatment after the pressing. According to a further embodiment, the coal particles can be subjected to a heat treatment after the impregnating step, before mixing with the water-containing binder system.
According to another embodiment, a pressed article, may contain up to 97% by weight of coal particles, and up to 12% by weight of components of a binder system, wherein, with respect to the weight of the coal particles as the material to be processed into pressed articles, it contains water-insoluble and/or water-repellent substances, or solid substances with water-repelling properties, in an amount of which the lower limit is 0.5% by weight, preferably 1% by weight, and the upper limit is 5% by weight, preferably 3% by weight, particularly preferably 2% by weight.
According to a further embodiment of the pressed article, the water-insoluble and/or water-repellent substance may belong to the group of substances comprising waxes, organic coking-plant or refinery products, as well as plastics or plastics scrap, and used oil. According to a further embodiment of the article, the pressed article also may contain iron- or iron-oxide-containing particles.
According to yet another embodiment, a pressed article as described above can be used in a process for producing pig iron in a fixed bed as a carbon carrier or in a process for producing carbon carriers for a process for producing pig iron in a fixed bed.
The method according to various embodiments is outlined below on the basis of the block diagrams represented in
According to various embodiments in a method for producing a pressed article containing coal particles in which the coal particles are mixed with a water-containing binder system and the mixture thereby obtained is further processed into pressed articles by pressing,
before the mixing with the water-containing binder system, the coal particles are subjected to an impregnating step in which they are impregnated with a substance.
During the impregnation, the substance either penetrates into the pores of the coal particles and, by filling the pore space, correspondingly prevents penetration of components of the aqueous binder system. Or the substance becomes deposited in the outlets of the pores on the coal particle surface, also known as pore necks, and, by this plugging of the pore necks, prevents penetration of components of the aqueous binder system into the pores.
In this way, aqueous binder system that is required on the coal particle surface for binding purposes is prevented from performing these binding purposes after penetration into the pores. Correspondingly, the amount of aqueous binder system that is required is reduced in comparison with a method in which aqueous binder system can penetrate into the pores.
Apart from water, the aqueous binder system may contain one or more further components. The impregnating step may comprise damping the coal particles with the substance, spraying the coal particles with the substance, mixing the substance into a moving packed bed of the coal particles, or mixing the substance into a fluidized bed of the coal particles.
According to one embodiment, the substance with which the coal particles are impregnated in the impregnating step is water. Then, in the impregnating step, water is sucked into the pores, which as a result no longer show any tendency to absorb components of the aqueous binder system fed to the coal particles after the impregnating step. As a result, components which, in the case of previous methods, were sucked into the pores, and consequently became ineffective for the binding of the pressed articles, can make a contribution to the binding of the pressed articles.
By limiting the proportion of pressed articles impregnated with water in a charging mixture for a pig iron production process in combination with carbon carriers which have a lower moisture content than these pressed articles, the amount of water introduced into the pig iron production process can be limited to an acceptable amount.
According to another embodiment, the substance with which the coal particles are impregnated in the impregnating step is a water-insoluble and or water-repellent substance.
If, in the impregnating step, the pores are filled with such a substance, and the pore walls are thereby coated with such substances, the tendency of the pores to absorb components of the aqueous binder system decreases. If the outlets of the pores on the carbon particle surface are closed by such substances, no components of the aqueous binder system can penetrate any longer into the pores. As a result, components which were previously sucked into the pores, and consequently became ineffective for the binding of the pressed articles, can make a contribution to the binding of pressed articles.
The water-insoluble and/or water-repellent substance preferably belongs to the group of substances comprising waxes, organic coking-plant or refinery products, as well as plastics or plastics scrap. It may also be used oil. The substances are usually available in large amounts at low cost.
In this case, the impregnating step advantageously takes place at a temperature at which the water-insoluble and/or water-repellent substance is liquid, particularly viscous. The liquids are regarded as viscous in this sense if their viscosity is at least 1 Pas, and at most 100 Pas, for example 10 Pas. Under these conditions, the substance is dispersed on the surface of the coal particle and penetrates into the outlets of the pores but scarcely into the interior of the pores. As a result, the consumption of the water-insoluble and/or water-repellent substance in the impregnating step is kept low. The water-insoluble and/or water-repellent substance advantageously solidifies in the outlets of the pores on the coal particle surface during cooling.
According to another embodiment, the substance with which the coal particles are impregnated in the impregnating step is an aqueous solution of a substance or a substance mixture. For example, it comprises molasses, an aqueous solution of a mixture of carbohydrates and other natural substances.
In principle, dissolved substances of all kinds that improve the hot strength and green strength of the pressed articles may be used, for example starch or lignin lyes from spent liquors of pulp production.
It is preferred to use solutions of substances or substance mixtures which are transformed into water-insoluble substances by heat treatment and/or reaction with the coal particles. This achieves the result that the effects induced by these substances or substance mixtures are not lessened by them dissolving in the water of the water-containing binder system and being washed out from the pores.
According to another embodiment, the substance with which the coal particles are impregnated in the impregnating step is an aqueous suspension of solid colloids, the solid substance having water-repelling properties. An example of this are suspensions of colloidal talc, of graphite or of waxes in water. If the solid substances are deposited in the pores or in the pore necks, it is made more difficult for water-containing binder systems to enter on account of the high surface tension of the water-repelling solid substances.
According to another embodiment, the substance with which the coal particles are impregnated in the impregnating step is an emulsion containing water on the one hand and carbon-containing substances on the other hand, such as for example bitumens, crude tars obtained from hard coal, pitches, waxes or oils.
When such emulsions penetrate into the pores, the carbon-containing substances are deposited in thin layers on the pore surface. During pyrolysis, carbon layers are produced from these thin layers. These reduce the reactivity of the pressed article with respect to hot CO2-containing gases in comparison with an embodiment in which no thin layers of the substances are deposited in the pores. The reason for this is that the carbon layers produced from the substances contain little or no substances that act catalytically with respect to the reaction with hot CO2-containing gases. By contrast, the coal particles or the material that is to be processed into pressed articles contain(s) catalytically acting compounds, for example iron or alkalis. Correspondingly, the reactivity of a pressed article of which the surface and pores are covered with a carbon layer created from the substances is less than that of a pressed article without such a carbon layer.
When using coal particles which require pre-drying before being processed into pressed articles, it is of advantage for commercial reasons not to pursue the drying to a moisture content much below 5% by weight, that is to say to a moisture content of at most 4% by weight. As a result, the creation of additional pore volume as a consequence of the drying is limited and correspondingly less substance is taken up by pores in the impregnating step. Correspondingly, less substance is used in the impregnating step. Moreover, less expenditure in terms of equipment and energy is required for the drying.
The lower limit of the amount of substance added in the impregnating step, known as impregnating agent, is 0.5% by weight, preferably 1% by weight; the upper limit is 5% by weight, preferably 3% by weight, particularly preferably 2% by weight, with respect to the weight of the material to be processed into pressed articles, that is to say the coal particles. Adding more than 5% by weight of impregnating agent is not economically advisable. Adding less than 0.5% by weight of impregnating agent means that impregnation is no longer effective.
According to one embodiment of the method, the binder system contains molasses and quicklime or hydrated lime. It may also consist of these components.
According to another embodiment, the binder system contains molasses in combination with strong inorganic acids, such as for example phosphoric acid, sulfuric acid or nitric acid.
According to one embodiment of the method, the binder system contains an emulsion of bitumen in water. It may also consist of such an emulsion.
According to further embodiments, the binder system contains products from spent liquors of pulp production, starches, cellulose, beet chips, waste paper pulp, wood pulp or other long-chained polyelectrolytes such as for example carboxy methylcellulose.
Since binder systems containing quicklime or hydrated lime have the disadvantage that quicklime CaO and hydrated lime Ca(OH)2 increase the reactivity of the pressed articles with respect to hot CO2-containing gases as a result of catalytic effectiveness, the embodiments without quicklime or hydrated lime have the advantage of providing pressed articles with comparatively lower reactivity.
According to one embodiment of the method, iron- or iron-oxide-containing particles are also processed in a mixture with the coal particles.
According to one particular refinement of the method, the pressed articles are subjected to a heat treatment after the pressing. The heat treatment takes place at a temperature that is increased in comparison with the pressing. The heat treatment brings about a drying and/or hardening of the pressed articles. The heat treatment may take place at temperatures of preferably ≧250° C. and ≦350° C., at which irreversible chemical processes can transform binder components. For example, water-soluble binder components may be transformed into water-insoluble compounds.
The compounds produced in such transformations may make a contribution to the strength of the pressed articles.
In the case of a binder system containing molasses, for example, the transformation of molasses takes place by caramelization.
According to one particular refinement of the method, the coal particles are subjected to a heat treatment after the impregnating step, before mixing with the water-containing binder system.
The heat treatment brings about a drying. For the case where there are solutions or emulsions in the pores, the heat treatment additionally brings about a concentration of the solutions, suspensions or emulsions, and correspondingly a coating of the pore walls with dissolved, suspended or emulsified components. In addition to the aqueous binder system then to be added, these may make a contribution to increased hot strength and green strength of the pressed articles.
Furthermore, the heat treatment may bring about the transformation of the coating of the pore walls, initially produced as a result of the heat treatment, into water-insoluble compounds, or into compounds lowering the reactivity of the coal particles with respect to hot CO2-containing gases. The maximum temperature of the heat treatment is restricted by the pyrolysis of the coal particles and is at 350° C. The lower limit for the temperature in this heat treatment is at 150° C.
If the same water-containing emulsion is used for the impregnation as is used as the water-containing binder system, the amount added in the impregnating step is less than the amount of water-containing binder system added in the subsequent mixing. For example when using a bitumen-in-water emulsion in the impregnating step and as the binder system, in the impregnating step an addition of 2-3% by weight is made, while 7-10% by weight are added later as the binder system. The same applies if the same aqueous solution of a substance or a substance mixture is used for the impregnation as that used as the water-containing binder system. For example when using molasses in the impregnating step and as the binder system, in the impregnating step an addition of 3 to 5% by weight is made, while 6 to 8% by weight are added later as the binder system. In this case, the limits of the specified ranges are also included. In these cases, a heat treatment is necessary after the addition in the impregnating step, in order to remove the water, as the carrier liquid, to the extent that the emulsified substances or the dissolved substances settle in the pores or the pore necks. As a result, the pores are covered or the pore necks are plugged. Altogether, therefore, less water-containing binder system is required for producing the pressed articles than in the case of production without an impregnating step.
The processing into pressed articles after the impregnating step can be performed by known methods, for example as described in WO 02/50219A1 or AT005765U1, or by any method suitable for processing coal particles with a water-containing binder system into pressed articles.
An addition of water-containing binder systems which, according to various embodiments, only takes place after the impregnating step with a water-insoluble and/or water-repellent substance, in the production of pressed articles reduces the costs of the method in comparison with conventional methods, such as for instance according to WO02/50219A1. The avoidance of water absorption by the coal during the production of pressed articles with water-containing binder systems on the one hand reduces the specific coal consumption in pig iron production methods in which the pressed articles or coke obtained from them are used, since less water from the binder system is present in the pressed article and correspondingly less energy has to be expended for vaporizing said water. On the other hand, when the method according to various embodiments is used, it is possible to dispense with a necessity that occurs in conventional methods for producing pressed articles for afterdrying the pressed articles as a result of the water absorption from the binder system, or it is possible to reduce the drying effort, resulting in an energy saving. Since it is correspondingly possible to dispense with the setting up or operation of devices for afterdrying, or it is possible to reduce the dimensions of the devices and the effort involved in their operation, this is synonymous with a reduction in operating costs and a reduction in investment costs.
Depending on the type of substance used for the impregnation, a lessening of the CO2 reactivity of the semicoke produced after pyrolysis of the pressed articles in a melter gasifier, or of the coke obtained from pressed articles, may be obtained as an additional advantageous effect of the impregnating step. A low CO2 reactivity is desired when operating a melter gasifier, in order that the semicoke in the fixed bed of the melter gasifier or the coke in the fixed bed of a blast furnace remains stable from the charging onto the bed surface to the reaching of the direct gasification zone in the region of the oxygen nozzles or the air-blast tuyeres and, as a result, promotes the permeability of the fixed bed with respect to the gas distribution and drainage of molten phases. The lessening of the CO2 reactivity of the semicoke or the coke is achieved by the inner surface of the pores of the coal particles in the pressed article no longer being able to be coated by the impregnation with a binder which contains reactivity-promoting substances. For example, the molasses as a binder component contains alkalis as reactivity-promoting substances. If coating of the inner surface of the pores with molasses is avoided by the impregnation, for example with substances containing bitumens or waxes, the CO2 reactivity is therefore lowered in comparison with semicoke or coke obtained by means of a method without an impregnating step.
A lower proportion of undersized coke is often added to the charging coal in the COREX® or FINEX® process for pig iron production in a fixed bed of a melter gasifier in order to improve the permeability of the fixed bed. When using pressed articles produced according to various embodiments, or coke produced in this way, softening of the semicoke or coke particles is inhibited by hot CO2, and consequently a disintegration of the particles is counteracted. With a packed fixed bed of pressed articles produced according to various embodiments from semicoke derived by pyrolysis, a much better gas permeability and a better drainage behavior of the fixed bed than according to the prior art is made possible. The improvement in the reactivity properties of the semicoke therefore makes it possible to reduce or even avoid the addition of coke to the COREX® or FINEX® charging coal.
In the area of coking plant technology, it is known to improve the quality of the coke produced from the charging coal by increasing its bulk density. The use of many charging coals for producing metallurgical coke is made possible in the first place by compacting of the charging coal. Apart from tamping coking plants, therefore, method variants for coking plants in loose-fill operation were developed, providing briquetting or partial briquetting of the charging coals. From today's viewpoint, however, briquetting with a bituminous binder is problematic for commercial reasons, hot briquetting or briquetting with binders originating from hard coal tar is problematic for health reasons, and briquetting with molasses or comparable binders is problematic because of the introduction of undesired substances into the coke.
The method according to various embodiments for producing pressed articles makes it possible to reduce binder consumption, or to mitigate the harmful effects of reactivity-promoting binder components, even when producing coke using articles pressed from the charge materials.
The pressed articles may be, for example, briquettes or compressed strips from compacting.
Pressed articles contain up to 97% by weight of coal particles, and up to 12% by weight of components of a binder system, as well as, with respect to the weight of the coal particles as the material to be processed into pressed articles, water-insoluble and/or water-repellent substances, or solid substances with water-repelling properties, in an amount of which the lower limit is 0.5% by weight, preferably 1% by weight, and the upper limit is 5% by weight, preferably 3% by weight, particularly preferably 2% by weight.
According to one embodiment, the pressed article also contains iron- or iron-oxide-containing particles. Such particles may, for example, originate from dusts or slurries occurring in the production of pig iron or steel.
According to
The method according to various embodiments as shown in
Represented in
| Number | Date | Country | Kind |
|---|---|---|---|
| A65/2009 | Jan 2009 | AT | national |
This application is a U.S. National Stage Application of International Application No. PCT/EP2009/067839 filed Dec. 23, 2009, which designates the United States of America, and claims priority to Austrian Application No. A65/2009 filed Jan. 16, 2009, the contents of which are hereby incorporated by reference in their entirety.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP09/67839 | 12/23/2009 | WO | 00 | 9/21/2011 |
